The present invention relates to a laser optical system that includes a laser source that emits a laser beam of a plurality of wavelengths (i.e., a multiline laser). Particularly, the present invention relates to a laser optical system used in an exposure device such as a laser photo plotter that plots a pattern on a photosensitive surface by scanning a single beam spot or a plurality of beam spots formed thereon.
The exposure device such as a laser photo plotter, which requires relatively large laser power, employs a gas laser. A gas laser generally emits a laser beam having a plurality of peak wavelengths. For example, an argon laser emits a laser beam having a plurality of peak wavelengths in ultraviolet and visible regions.
In such an exposure device, it is desirable to increase plotting speed to increase productivity. In order to increase the plotting speed, it is necessary to increase scanning velocity or it is necessary to divide a laser beam to form a plurality of beam spots on an object surface to be exposed, which requires large laser power to maintain a predetermined light amount of the single beam spot.
Therefore, it is desirable to use a plurality of peak wavelengths of a laser beam emitted from a gas laser to keep the large laser power. Particularly, the exposure device used for manufacturing a printed circuit board (PCB) or a semiconductor device has higher requirement in using a plurality of wavelengths. Because, although the fine circuit pattern requires a high resolution with short wavelength laser beam such as ultraviolet beam, luminous efficiency of the gas laser in ultraviolet region is lower than that in the visible region, a single peak wavelength in the ultraviolet region may not supply sufficient laser power for the high speed plotting.
However, when a plurality of wavelengths are used in the optical system that is not corrected in lateral chromatic aberration, a convergent point of one peak wavelength is slightly deviated from convergent points of other peak wavelengths on the photosensitive surface. This enlarges the diameter of the resultant beam spot, thereby lowering the plotting performance of the exposure device.
Particularly, when a plurality of wavelengths are used in the optical system that includes a diffractive element instead of or additional to conventional refractive elements, the plotting performance of the exposure device would be seriously lowered because of larger wavelength dependence of the diffractive element. The diffractive element may be an acoustooptic modulator (AOM), a diffractive beam-dividing element, or the like.
Therefore, the conventional optical system that employs a gas laser and a diffractive element, is provided with a filter that selects one peak wavelength. The filter cuts the peak wavelengths other than the selected peak wavelength, which lowers energy efficiency.
It is therefore an object of the present invention to provide a laser optical system including a multiline laser source and a diffractive element such as an AOM or a diffractive beam-dividing element, which is capable of increasing energy efficiency while keeping the plotting performance.
For the above object, according to the present invention, there is provided an improved laser optical system, which separates a laser beam emitted from a multiline laser source into monochromatic beams by a wavelength separating element. The laser optical system modulates intensity of the respective monochromatic beams by respective modulating optical systems that include diffractive elements, and then combines the modulated monochromatic beams by a beam combining optical system. The combined monochromatic laser beams may form a common beam spot or separate beam spots on an object surface to be exposed.
With this construction, since each modulating optical system treats a monochromatic beam, chromatic aberration due to the wavelength dependence of the diffractive element presents no problem. As a result, when the laser optical system is provided with a pair of the modulating optical systems for example, two peak wavelengths of a laser can be used, which increases the energy efficiency. Further, when the modulating optical systems are optimized for the respective wavelengths, the resultant beam spot can be kept small even if the combined monochromatic beams form a common beam spot on the object surface, which keeps a high plotting performance.
The modulator may be an acoustooptic modulator (AOM) that changes a direction of an emergent beam by diffraction caused by input ultrasonic wave. Either a diffracted beam or a non-diffracted beam emerges from the modulator as a modulated beam to form the beam spot on the object surface. In this case, the modulator itself comprises the diffractive element. The diffractive element may be the modulator or an element included in addition to the modulator.
Further, the modulating optical system may include a diffractive beam-dividing element that divides the incident monochromatic beam into a plurality of separate beams. In this case, the modulator should be a multichannel modulator, which may be an AOM or other modulator. It is desirable that the incident beam on the diffractive beam-dividing element is a parallel beam, while the channels of the multichannel modulator should be located at convergent points of laser beams. Therefore, a condenser lens or a condenser mirror is required to be located between the diffractive beam-dividing element and the multichannel modulator.
When the modulating optical systems include the diffractive beam-dividing elements, each of the monochromatic beams forms a plurality of beam spots on the object surface. In this case, the beam spots of one monochromatic beam may be overlapped with corresponding beam spots of other monochromatic beams to form common beam spots on the object surface. On the other hand, all of the beam spots may be separated from one another.
To overlap the beam spots of different wavelengths (i.e., to overlap the beam spots emerged from different modulating optical systems), pitches of the beam spots of respective wavelengths must be identical to form common beam spots. Further, even if the respective monochromatic beams form separate beam spots, it would be desirable that pitches of the beam spots of respective wavelengths are identical. There are at least two ways to keep the identical pitches of the beam spots.
In the first way, the relationship among the grating pitches of the diffractive beam-dividing elements of the respective modulating optical systems are determined such that the grating pitch is proportional to the wavelength. In the first way, the focal lengths of the condenser lenses are identical.
In the second way, the relationship among the focal lengths of the condenser lenses of the respective modulating optical systems are determined such that the focal length is inversely proportional to the wavelength. In the second way, the grating pitches are identical.
Further, when the respective monochromatic beams form separate beam spots, the beam spots of respective wavelengths are preferably alternately aligned along a straight line. Namely, it is desirable that all of the beam spots are arranged along a straight line in such a fashion that one beam spot of one wavelength is positioned next to one beam spot of another wavelength.
The wavelength separating element may separate the laser beam from the multiline laser source into two beams of different wavelengths. In such a case, a pair of modulating optical systems should be provided and the beam combining optical system may be a polarizing beam splitter. When the polarizing beam splitter is used, the laser beam modulated by one modulating optical system should be incident on the polarizing beam splitter as S-polarized light and the laser beam modulated by the other modulating optical system should be incident as P-polarized light.
Still further, the laser optical system of the embodiment includes a deflector for deflecting a plurality of laser beams combined by the beam combining optical system, and a converging optical system that converges the laser beams deflected by the deflector to form beam spots on the object surface.